With the advent of an energy saving era, electric bulb type fluorescent lamps and compact type fluorescent lamps are becoming prevalent as energy saving light sources for replacing general electric bulbs in the field of lighting.
Conventionally, there have been efforts to produce smaller-scale electric bulb type fluorescent lamps with the same outer shape as general incandescent light bulbs. In recent years, double spiral glass tubes have been applied to the arc tubes for such fluorescent lamps since they have advantages for producing smaller scale lamps.
Such a double spiral glass tube is formed by bending a glass tube at a central portion thereof, and winding the two portions of the glass tube, which extend from the central portion, around a spiral axis. After this, a pair of electrode assemblies are attached to both ends of the glass tube, respectively, to hermetically seal the glass tube. Mercury and a buffer rare gas such as argon are then hermetically inserted into the inner space of the glass tube. A phosphor is preliminarily applied to the inner wall of the glass tube. As the phosphor is excited by ultraviolet light emitted from mercury atoms, visible light is emitted from the arc tube. The typical measurement of the glass tube is, for example, as follows in the case of a 22 W fluorescent lamp for replacing a general 100 W electric bulb: a tube inner diameter di is 7.2 mm, and a tube thickness t is 0.9 mm. Here, in general, a glass tube whose thickness is in a range from 0.7 mm to 1.2 mm is used for the arc tube of the electric bulb type fluorescent lamp, when the tube inner diameter di is in a range from 5.0 mm to 10.0 mm. Important factors for determining the measurement are, especially, the easiness in performing the forming process and the mechanical strength.
The electric bulb type fluorescent lamp is completed as a final product when the arc tube is housed in an outer glass bulb, and a base is attached to an assembly of an electronic ballast with the outer glass bulb containing the arc tube.
Meanwhile, it is well known that the lamp efficiency of a general fluorescent lamp depends on the mercury vapor pressure inside the tube, where the mercury vapor pressure is uniquely defined by temperature Tc at the coldest point portion Sc of the arc tube, and that the best lamp efficiency is obtained when the temperature Tc is in the most appropriate range of 55° C. to 65° C. when the tube inner diameter di is in the range from 5.0 mm to 10.0 mm. Also, in general, when an ordinary electric bulb type fluorescent lamp is lighted constantly, the coldest point temperature Tc of the arc tube housed in the outer glass bulb becomes higher than the most appropriate range.
In producing some actually sold electric bulb type fluorescent lamps to which the double spiral glass tube has been applied, the central portion at which the glass tube is bent is expanded (see Document 1, for example), a bump projecting from the central portion is formed, and the coldest point portion Sc is set at the bump. Further, there is known a method having been developed and introduced, in which the coldest point portion Sc is connected with the outer glass bulb by silicone resin being a thermal conductor so that the portion can be cooled. This method enables the coldest point temperature Tc at the coldest point portion Sc to be reduced to the most appropriate range that provides the best lamp efficiency.
The bump of the double spiral glass tube is formed as follows, for example. First, a bump forming section within the central portion of the glass tube is heated using a gas burner to become softened, and a gas is injected into the glass tube to expand a portion around the softened bump forming section (see Document 2, for example).
Document 1: Japanese Patent Application Publication No. 2003-173760
Document 2: Japanese Patent Application Publication No. 2004-87397